Thin film transistor liquid crystal displays (TFT-LCDs) are a kind of the major LCDs. They adopt the TFT technology for improving the image quality.
TFTs are a kind of field-effect transistors. The general fabrication method is to deposit various thin films, such as semiconductor active layers, dielectric layers, and metal electrode layers, on a substrate. The silicon layers, including amorphous silicon (a-Si) or polysilicon (poly-Si), in TFTs are fabricated by mainly using silicide gas.
Contrast to poly-Si TFTs, using a-Si TFTs to manufacture displays can reduce the production costs. Besides, large-area fabrication on glass substrate at low temperatures is feasible for a-Si TFTs, which improves the production rate. Nonetheless, the characteristics of a-Si TFTs are easily influenced by temperature. With the same gate voltage, if the temperature is higher, the current flowed through the drain and the source are large. Conversely, if the temperature is lower, the current flowed through the drain and the source are small. Since a-Si TFTs are used as the driving switches to control the display status of the images in the display, temperature will influence the contrast and the gamma curve of the images.
Due to the variation in TFT characteristics as the temperature changes, several solutions for solving the image problem in the display are published in US patents as below.
In the U.S. Pat. No. 7,696,977, an apparatus for driving display panel with temperature compensated driving voltage is disclosed. The apparatus mainly comprises a temperature sensor, a temperature section register, a plurality of comparing units, a voltage register, a voltage controller, and a driver.
The operation of this circuit is described as follows. The temperature sensor senses the temperature and outputs the temperature data. The plurality of comparing units compare the temperature data outputted by the temperature sensor with the temperature section data stored in the temperature section register for outputting comparison data having a predetermined pattern of bits. The voltage controller receives the comparison data and selects the voltage data corresponding to the comparison data for outputting the voltage control signal. The driver receives the voltage control signal for outputting the driving signal to the display panel. In other words, after the temperature sensor senses the temperature, the driver can output diving voltages with different levels for driving the display panel according to different temperatures through the temperature section register, the comparing units, and the voltage controller.
The circuit architecture of temperature compensated driving voltage according to the patent changes the ideal driving voltage for the liquid crystals according to the characteristics of the liquid crystals at different temperatures. In order to detect the temperature of the panel, it is required to dispose a plurality of temperature sensors at the periphery of the panel, which needs more costs on purchasing ICs. The circuit architecture according to the patent requires complex circuit including the temperature section register, the comparing units, and the voltage controller. If the circuits and the panel are fabricated on the same glass for saving the costs of ICs, the area of the layout for the circuits will be too large and difficult to be applied to narrow-frame displays.
Moreover, a display driving circuit having temperature compensation circuit is disclosed in the U.S. Pat. No. 7,038,654. According to this circuit architecture, after the temperature sensor senses the temperature, the driving circuit can adjust automatically the driving voltages for the liquid crystals at different temperatures through the control circuit, the reference voltage circuit, the step-up circuit, and the comparator.
According to the temperature sensor of the patent, a voltage is outputted to two diodes (D1 and D2) coupled in series with a current source via an operational amplifier OP1 and two resistors (R1 and R2). Because the voltage drops across the diodes are changed according to the change of the temperature, the voltage outputted to an operational amplifier OP2 will be different according to the change of the temperature. Nonetheless, the method of allowing DC current flowing through the two diodes results in larger static power consumption. In addition, for saving the IC costs, fabricating this circuit according to the patent and the panel on the same glass requires larger layout area for circuit and consuming more power.
Accordingly, the present invention provides a temperature sensing circuit and a driving circuit for LCDs to have better image quality at different ambient temperatures.